1. Field of the Invention
This invention relates to the field of converting molten sulfur (or sulphur) into solid sulfur granules using sulfur seeds.
2. Description of the Related Art
Sulfur is an important industrial commodity, most commonly produced in molten liquid form as a byproduct from oil and gas refining. Much of the liquid sulfur is solidified into various “forms,” such as granules, pastilles or prills for ease in transportation and use. The various forms are commercially produced by different processes. Granules are produced by enlarging “seeds” in a granulating drum; pastilles are formed by laying sulfur drops onto a continuous stainless steel belt; and prills are produced by dropping liquid sulfur into a bath of cooling water. Whereas pastilles and prills are produced by solidifying single sulfur droplets, the production of granules requires a “seed” particle to initiate the enlargement process.
A criteria for evaluating sulfur product was established by the Sulphur Development Institute of Canada (SUDIC). The shape and particle size distribution of sulfur forms under the criteria is generally spherical with the diameter between 2 mm to 6 mm. Sulfur forms qualify as “premium product” or “standard product” depending on shape, particle size distribution, moisture content, and friability. Sulfur granules and pastilles meet the premium product specification in all respects. Wet prills do not meet the premium product specification with respect to moisture, and are considered “standard product.” A sulfur seed is understood in the industry to be a sulfur particle that requires further enlargement to become a sulfur granule and obtain maximum commercial value. A sulfur seed is generally considered to be smaller than 2 mm in diameter.
The three commercial forming processes also differ in the manner in which heat is removed to effect sulfur fusion and cooling of solid particles. In drum granulation, sulfur is cooled by transferring heat to the atmosphere inside the drum, the temperature of which is moderated by evaporation of water droplets sprayed into the drum. Pastilles are cooled by spraying water to the underside of the stainless steel belt, which in turn is cooled by evaporation in a cooling tower. Wet prills are cooled by transferring heat to the water bath which in turn is cooled by evaporation in a cooling tower.
U.S. Pat. No. 4,213,924 (Shirley) proposes a method for producing sulfur granules in a rotary drum having lifting flights to elevate the seeds that then fall from the flights as curtains which are then coated with a spray of liquid sulfur. The discharged product from the drum is screened, and seeds that have not been adequately enlarged are returned on conveyors and either cooled or heated before being recycled into the input end of the drum. The '924 Shirley patent also proposes crushing oversized product discharged from the granulating drum and recycling the crushings to the drum as seed or recycle material. A disadvantage with crushing is that dust is created that may become released into the environment. The dust may be explosive and/or a health hazard. Also, the crushings are not uniform in size or spherical in shape.
In the past, fans have been proposed to force circulation of air through the falling curtains for enhanced cooling. A cooler sulfur product tends to be less friable and less susceptible to “caking” or “agglomerating” in storage. However, the fans may become unbalanced from the sulfur that accumulates on the blades.
U.S. Pat. No. 4,272,234 (Tse) proposes the production of sulfur seeds in a granulating drum by raising the temperature of the rotating bed of sulfur particles for a short period of time. The sulfur sprayed on the falling particles in a particular zone of the drum is proposed to not immediately solidify but remain soft or plastic on the particles' surface, and when the particles are tumbled in the bed, the abrading action of the other particles are proposed to break off small pieces of the soft coating having a diameter in the range of about 0.1 to about 1.0 mm.
U.S. Pat. No. 4,507,335 (Mathur) proposes the generation of sulfur seed particles inside a granulating drum in certain controlled conditions in which liquid sulfur droplets found in the outer edges of a thin, flat spray plume solidify into seeds prior to contacting the falling curtain of solid sulfur particles. U.S. Pat. No. 5,435,945 (De Paoli et al.) proposes creating sulfur seeds within a granulating drum by intersecting the molten sulfur spray with a water spray or by creating a spray of sulfur droplets that are allowed to solidify in the atmosphere within the granulating drum.
A disadvantage of producing seeds in a granule enlargement drum is that the conditions required in the drum for optimum granule production are not the same conditions required for optimum seed production. It generally takes a skilled technician to monitor and operate the system.
U.S. Pat. No. 7,638,076 (Koten) proposes inter alia, passing molten sulfur through a nested strainer, a drip tray with a heating channel, an injection conduit for delivery of a cooled zone of water to create solid prills, and thereafter moving the prills through a stationary curved screen and a vibrating screen.
A need exists for a method and system to more efficiently create sulfur seeds to be used for enlargement into sulfur granules. It would be desirable to control the size distribution and production rate of seeds in a manner that corresponds directly to enlargement requirements to enable sulfur granules to be produced in a one pass continuous enlargement process through a granulating drum at a reasonably high production rate, thereby substantially eliminating the need for screening the drum output and recycling undersized product with conveyors back to the drum input end. A need also exists to improve the rate at which granules are cooled in the drum in order to realize improved product quality and higher production rates.